1. Technical Field
The present principles relate to the principle of redundant time-staggered content transmission to enhance robustness. This technique is termed “staggercasting”. More particularly, they relate to a power savings method and apparatus for use in a staggercasting environment.
2. Description of the Related Art
Current digital television transmission standards in the United States, as proposed by the Advanced Television Systems Committee (ATSC) dated Sep. 16, 1995, use a single carrier modulation technique: eight level vestigial sideband modulation (8-VSB). Because The 8-VSB signal is susceptible to signal degradation in the communications channel, such as fading caused by multipath and other signal attenuation. While some such fading may be compensated by channel equalization techniques, if the fade is long enough and severe enough, then the receiver will lose the signal and the demodulator system will lose synchronization. Reacquiring the signal, and resynchronizing the demodulator can take several seconds and is quite objectionable to a viewer.
To overcome this problem, a first ATSC proposal permits creation of a second communications channel by permitting use of a more robust modulation technique for a limited period of time, e.g. less than 10%. For example, a 2 or 4-VSB modulation technique may be used for selected frames. A second ATSC proposal permits a more robust encoding technique, e.g. trellis encoding, while maintaining an 8-VSB modulation technique. Such a system permits improved performance with compatible receivers while maintaining backwards compatibility with existing receivers.
Another technique for overcoming fading is staggercasting [references from our earlier patent applications]. Staggercasting communications systems transmit a composite signal including two component content representative signals: one of which is offset in time with respect to the other. Put another way, one of the component content representative signals is advanced with respect to the other. The composite signal is broadcast to one or more receivers through a communications channel. At a receiver, the advanced-in-time component content representative signal is delayed through a delay buffer so that it becomes resynchronized in time with the other component content representative signal. Under normal conditions, the undelayed received component content representative signal is used to reproduce the content. If, however, a signal fade occurs, then the previously received and advanced-in-time content representative signal in the delay buffer is used to reproduce the content until either the fade ends and the composite signal is available again, or the delay buffer empties. If the delay period, and the associated delay buffer, is large enough then most probable fades may be compensated for. As such, it will be apparent that staggercasting allows the receiver to substitute or reconstruct lost portions of a main signal with a correctly received time-staggered supplemental signal.
Staggercasting improves robustness of reception of a terrestrially transmitted signal by overcoming fades. As mentioned above, the method calls for substituting or reconstructing faded (and hence lost) segments of a program from a time-advanced version of the program that was correctly received. However, in order to process and store both versions of the program (i.e., both the main/normal signal and the supplemental/staggercast signal), additional processing power and memory for storage is required.
Thus, it would be desirable to provide a power savings system that addresses this increased power requirement in a staggercasting scenario.